The invention relates to a belt retractor for a vehicle safety belt.
Conventional belt retractors comprise a belt spool, at least one locking toothing into which a locking catch can be caused to engage, at least one force limiter which can make possible a rotation of said belt spool in an unwinding direction of a belt webbing.
The force-limiting device serves to make it possible, with a belt spool which in itself is blocked, that a certain amount of belt webbing can be unwound from the belt spool against a determined torque acting as resistance. In this way, an additional way is made available for the deceleration of a vehicle occupant, whereby the maximum traction acting in the belt webbing can be limited to an extent at which no injuries to the vehicle occupant occur. A switchable clutch may be provided in order to adapt the characteristic of belt webbing unwinding force versus unwound belt webbing to the respective conditions. For example, with a heavy vehicle occupant it is desirable that a comparatively high force is necessary for belt webbing unwinding, whereas with a light vehicle occupant the unwinding of the belt webbing must already be possible with a smaller traction, so that the force-limiting effect occurs and injuries to the vehicle occupant are prevented. For this purpose, hitherto belt retractors have been known, in which a torsion rod is used as force limiter, which consists of several individual parts. The clutch makes it possible to connect the individual parts of the torsion rod in parallel so that the characteristic of resistant torque or belt webbing unwinding force versus unwound belt webbing can be altered. Independently of the respective engagement state of the clutch, however, a part of the force limiter is effective in any case.
The invention provides a belt retractor for a vehicle safety belt, which comprises a belt spool, a locking catch, at least one locking toothing into which the locking catch can be caused to engage, and at least one force limiter which allows a rotation of the belt spool in an unwinding direction of the safety belt against a determined torque. In addition, a clutch is provided which cooperates with the force limiter in order to alter the determined torque. This gives greater freedom as regards the characteristic of belt webbing unwinding force versus unwound belt webbing using various constructions.
If only a single force limiter is present, which is deactivated with an actuated clutch, provision is made that the locking toothing is constructed on a locking disc which is connected non-rotatably with the belt spool when the clutch is actuated. In this design, the belt retractor acts like a conventional belt retractor without force limiter, when the clutch is actuated, the belt spool is therefore blocked by means of the locking toothing and an unwinding of the belt webbing from the belt spool is not possible. This type of operation can be used in particular after a first impact of the vehicle. With the so-called primary impact of the vehicle, the restraining effect for the vehicle occupant is usually provided both from the belt retractor with active force limiter and also from a gas bag. The force limiter is coordinated with the gas bag such that as uniform and gentle a restraining effect as possible is achieved. Of course, if a secondary impact occurs, the gas bag is no longer effective. If in this case the force limiter of the belt retractor were still active, an excessively great forward movement of the vehicle occupant would be possible, which could finally lead to contact with parts of the interior of the vehicle. For this reason, it is desirable that the belt retractor operates in a secondary impact in a conventional manner without a force limiter, i.e. the belt spool is blocked so that the belt webbing can not be unwound from it further. It is basically conceivable to switch the clutch mechanically, so that the force limiter in each case is deactivated and the belt spool can be completely blocked. This possibility is relevant for the case where the gas bag present in a vehicle is deactivated from the outset for every case, when for example the vehicle occupant sees an undesired potential for danger in such a gas bag. Because then the gas bag is no longer available for restraining the vehicle occupant, the forward movement of the vehicle occupant must be limited, so that the force limiter can not be active.
The design according to the invention also makes it possible to provide two force limiters which in the initial state with non-actuated clutch are connected in parallel and one of which is deactivated by actuating the clutch. In the initial state, the force necessary for unwinding belt webbing from the belt spool is determined by the sum of the holding moments provided by the two force limiters. If, on the other hand, the clutch was actuated, the characteristic results from the holding force provided by the remaining force limiter. The clutch can be actuated for example as a function of the weight of the corresponding vehicle occupant. If a suitable sensor arrangement detects, for example by means of pressure sensors in the vehicle seat, that the corresponding vehicle occupant has a low weight, then the clutch is actuated, so that only a single force limiter is active. In this case, an unwinding of belt webbing from the belt spool is already possible with smaller belt webbing forces.
According to a preferred embodiment of the invention, provision is made that the clutch has a carrier which is provided with carrier plates, which engage into a suitably shaped mounting of an output element, so that a form-fitting connection is achieved, and that a piston is provided, which in the non-actuated position supports the carrier plates, so that the carrier and the output element are connected non-rotatably with each other, and which in the actuated position frees the carrier plates, so that the connection between the carrier and the output element is released above a predetermined moment of torsion. This design leads to a particularly compact clutch which can be actuated in a reliable manner.
Preferably provision is made that the piston closes a pressure chamber in which a pyrotechnic charge is arranged, which on activation moves the piston from the non-actuated position into the actuated position. In this way, it is ensured that the clutch can be transferred reliably into the actuated position under all circumstances.
Basically all types of force limiters come into consideration As force limiters, for example a torsion rod or a band extraction device, in which a steel band guided in a guideway can be drawn out therefrom, whereby the holding force is provided owing to the friction between the steel band and the guideway.
Advantageous developments of the invention will be apparent from the sub-claims.